A conventional optical time-division multiplexing (OTDM) system is described, for example, in Proceedings of the 21st European Conference on Optical Communications (ECOC '95-Brussels) Th. A. 3. 5, pp. 983-986. An optical transmitter described in this publication has an optical signal (clock) generator which includes a laser diode and an optical electro-absorption (EA) modulator and generates a 10 Gbit/s pulse beam.
In the conventional transmitter, an RZ (return-to-zero) pulse train signal from the optical signal generator is split into two signals via an optical coupler. Then the split signals are modulated by an LiNbO.sub.3 modulator. After one of the modulated signals is delayed, both signals are multiplexed via another optical coupler.
There are some problems with the conventional optical time-division multiplexing transmitter module. Since the conventional transmitter employs an LiNbO.sub.3 modulator for modulating the optical signals, it is difficult to minimize the overall module size. In the case of employing a relatively smaller EA modulator instead of the LiNbO.sub.3 modulator, connections between its input/output ports and other components require optical fibers, which prevents further size reduction. Moreover, especially in the case of a multi-tap module with a PLC substrate (a planer lightwave circuit formed on a semiconductor substrate), it is difficult to align optical axes of EA modulator chips and the PLC substrate.